1. Field of the Invention
The present invention relates to an active-matrix display apparatus using light emitting devices as pixels and a method of driving the apparatus. Also, the present invention relates to an electronic system including such a display apparatus.
2. Description of the Related Art
In recent years, light-emitting flat display apparatuses using organic EL devices as light-emitting devices have been widely developed. The organic EL device is a device using a phenomenon in which an organic thin film emits light when an electric field is impressed on the film. The organic EL device is a low-power consumption device, because the device is driven by applying a voltage of 10 V or less. Also, the organic EL device is a self-emitting device emitting light by itself, and thus needs no lighting member, making it easy to save weight and to reduce thickness. Furthermore, the organic EL device has a very high response speed of about a few μ seconds, and thus has no afterimage at the time of displaying moving images.
Among the light-emitting flat display apparatuses using organic EL devices as pixels, in particular, active-matrix display apparatuses formed by the integration of thin-film transistors for individual pixels as driving devices are widely developed. The light-emitting flat display apparatuses of an active-matrix type have been disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, 2004-093682.
FIG. 20 is a circuit diagram schematically illustrating an example of an active-matrix display apparatus of the related art. The display apparatus includes a pixel array section 1 and a surrounding drive section. The drive section includes a horizontal selector 3 and a write scanner 4. The pixel array section 1 includes a column of signal lines SL and a row of scanning lines WS. Pixels 2 are disposed at intersections of individual signal lines SL and scanning lines WS. In the figure, in order to make it easy for understanding, only one pixel 2 is shown. The write scanner 4 includes a shift register, operates in response to a clock signal ck supplied from the outside, and transfers a start pulse sp, which is also supplied from the outside, in sequence, and thus outputs a control signal onto the scanning line WS in sequence. The horizontal selector 3 supplies a video signal onto the signal lines SL in accordance with line progressive scanning of the write scanner 4.
The pixel 2 includes a sampling transistor T1, a driving transistor T2, a holding capacitor C1, and a light emitting device EL. The driving transistor T2 is a P-channel type, the source thereof is connected to a power source line, and the drain thereof is connected to a light-emitting device EL. The gate of the driving transistor T2 is connected to the signal line SL through the sampling transistor T1. The sampling transistor T1 becomes conductive in response to the control signal supplied from the write scanner 4, samples the video signal supplied from the signal line SL to write the signal into a holding capacitor C1. The driving transistor T2 receives the video signal written in the holding capacitor C1 as a gate voltage Vgs, and causes a drain current Ids to flow to the light emitting device EL. Thereby, the light emitting device EL emits light at a luminance in accordance with the video signal. The gate voltage Vgs indicates the gate potential in reference to the source.
The driving transistor T2 operates in a saturation region, and a relationship between the gate voltage Vgs and the drain current Ids is expressed by the following characteristic expression:Ids=(½)μ(W/L)Cox(Vgs−Vth)2 
where μ represents the mobility of the driving transistor, W represents the channel width of the driving transistor, L represents the channel length of the driving transistor, Cox represents the gate capacitance of the driving transistor, and Vth represents the threshold voltage of the driving transistor. As is apparent from this characteristic expression, when the driving transistor T2 operates in the saturation region, the driving transistor T2 functions as a constant current source supplying the drain current Ids in accordance with the gate voltage Vgs.
FIG. 21 is a graph showing a voltage/current characteristic of the light emitting device EL. An anode voltage V is shown on the horizontal axis and the drive current Ids is shown on the vertical axis. In this regard, the anode voltage of the light emitting device EL is the drain voltage of the driving transistor T2. The voltage/current characteristic of the light emitting device EL changes over time, and the characteristic curve has a tendency of falling down with the elapse of time. Thus, even if the drive current Ids is constant, the anode voltage (drain voltage) V changes. On this point, in the pixel circuit 2 shown in FIG. 20, the driving transistor T2 operates in a saturation region, and thus allows the drive current Ids to flow in accordance with the gate voltage Vgs regardless of variations of the drain voltage. Accordingly, it is possible to keep the luminance of the light emission by the light emitting device EL at a constant regardless of a change in the characteristic of the light emitting device EL over time.
FIG. 22 is a circuit diagram illustrating another example of a pixel circuit of the related art. The different point from the pixel circuit of FIG. 20 shown before is that the driving transistor T2 has changed from a P-channel type to an N-channel type. It is often advantageous that all the transistors included in a pixel should be a N-channel type in view of the manufacturing process of the circuit.